An embodiment of an illumination system of the kind set forth is known from US2007/0008258. That document discloses an illumination based communication network making use of free space transmission of data incorporated in the light emitted by the light modules of the illumination system as intensity modulations. The lighting modules comprise a plurality of light emitting diodes (LEDs) each emitting light of different wavelengths (primary colors) and provide illumination usually requested from such systems for aiding visibility of objects to the human eye. Moreover, the network comprises a plurality of detectors each comprising multiple wavelength selective light-receiving elements (using band-pass filters) corresponding to the primary colors emitted by the LEDs.
The light modules transmit the data by (i) dividing a series of input signals into a plurality of signals corresponding to the different wavelengths of the LEDs, (ii) feeding each of the signals to the corresponding LED, (iii) performing code-division multiple access communication by modulating the emission intensities of the LEDs.
Receiving such modulated light allows a detector to (i) generate signals from each of the wavelength selective light receiving elements, (ii) correlate the signals with the spread code used for data broadcasting by the light modules, (iii) descramble the signals and regenerate the data as an output on an output port of the detector.
A drawback of the solution described in US2007/0008258 is that the human eye is very sensitive for intensity variations, in particular in certain frequencies ranges. Thus, the secondary use of an illumination system as part of a communication network by implementing data in the light emitted as intensity variations may diminish the primary function of the system as an aid to human vision, especially when used in the high sensitivity frequency ranges of the human eye. Therefore, a clear need exists to provide an alternative modulation scheme allowing data to be transmitted without (or at least at a highly reduced rate of) decreasing the perceived performance of the primary function of an illumination system over wide modulation frequency ranges.